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Coronary Heart Disease

2004 Edition

Stopping Fatal Heart Attacks With Fish
Eating foods rich in omega-3 fatty acids may reduce therisk of dying from coronary heart disease.

A line of research cast in the late 1970s has spawned a wealth of knowledge about a beneficial type of fat: omega-3 fatty acids. Found in fish and some plant products, these fats have been shown to lower the risk of sudden death from heart attacks in people with CHD. They may also benefit people at risk for CHD.

What Are Omega-3 Fatty Acids?

The three major types of highly unsaturated omega-3 fatty acids are eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), and alpha-linolenic acid (ALA). The only sources of EPA and DHA are fish (particularly fatty fish) and fish oils. (For a list of fish high in EPA and DHA, see the inset box on the opposite page.) ALA is found in certain plant and plant products, such as canola, soybeans, walnuts, and flaxseed, as well as the oils of these plants. Evidence is mounting that EPA and DHA can protect the heart; the data for ALA are not as strong.

The Evidence

Research into the heart benefits of omega-3 fatty acids began in the 1970s and 1980s when studies suggested that people who ate copious amounts of fish—for example, Japanese living in fishing villages and Greenland Eskimos—tended to have lower rates of atherosclerosis and heart attacks than Americans. Researchers hypothesized that the omega-3 fatty acids in fish might be responsible for this benefit.

A number of randomized clinical trials have examined the heart benefits of omega-3 fatty acid consumption in people diagnosed with CHD. A meta-analysis of these trials published in the American Journal of Medicine in 2002 reported that, in 11 randomized trials containing 15,806 people, omega-3 fatty acid-enriched diets or supplements decreased the risk of fatal heart attacks and sudden death from cardiac arrest by 30% and overall deaths by 20% compared with control diets or a placebo. However, the risk of nonfatal heart attacks was not significantly reduced.

How Do Omega-3 Fatty Acids Work?

Many experts suspect that the main mechanism by which omega-3 fatty acids reduce the risk of sudden death from CHD is by decreasing the heart’s susceptibility to dangerous arrhythmias (irregular heart rhythms). The heart can begin to beat irregularly when it is subjected to stress, for example when it is not getting enough oxygen. These irregular heartbeats may lead to fatal ventricular fibrillation. Omega-3 fatty acids are incorporated into the membranes of cells in the heart, where they may stabilize the heart’s rhythm.

Omega-3 fatty acids appear to have other beneficial effects on the heart. There is good evidence that they help reduce blood triglyceride levels. There is also some evidence, albeit less certain, that omega-3 fatty acids reduce the risk of blood clots, the growth of plaque in the arteries, high blood pressure, and inflammation, all of which contribute to atherosclerosis. They also appear to improve the function of blood vessels.

Getting Enough Omega-3 Fatty Acids

The average person in North America eats fish about once every 10 days and consumes about 1,600 mg of omega-3 fatty acids daily (about 200 mg of EPA and DHA combined and 1,400 mg of ALA). By contrast, the American Heart Association recommends that people not diagnosed with CHD eat fish, particularly those high in omega-3 fatty acids, at least twice weekly in addition to oils, nuts, and seeds high in ALA. For people who already have CHD, the organization recommends about 1,000 mg of EPA plus DHA per day from fatty fish, fish oil supplements, or a combination of both.

How the fish you eat is prepared may determine how well it protects against fatal CHD. In a study published in Circulation in March 2003, researchers found that people who ate broiled or baked fish three or more times per week had a 49% lower risk of fatal CHD and a 58% lower risk of death from a CHD-related arrhythmia than people who ate fish less than once a month. But people who ate fried fish or fish sandwiches experienced no reduced rate of CHD. Blood levels of omega-3 fatty acids were significantly higher in the people who consistently ate broiled or baked fish than in those who ate fried fish or fish sandwiches regularly. The researchers suggest that frying may reduce the omega-3 fatty acid content of fish. Another possible explanation is that the type of fish typically fried or used in fish sandwiches is not rich in omega-3 fatty acids.

Although current recommendations are that omega-3 fatty acids should preferably be obtained through food, some people may prefer fish oil supplements instead of, or in addition to, fish. Always consult your physician before beginning to take a fish oil supplement. Also, check the label of the supplement to determine its EPA and DHA content. A 1,000-mg fish oil capsule usually contains about 180 mg of EPA and 120 mg of DHA. This means that a person may have to take two to three capsules daily, in addition to eating fish regularly, to meet the American Heart Association’s recommendation for omega-3 fatty acid intake for people with CHD.

Unfortunately, because the U.S. Food and Drug Administration does not regulate these supplements, different batches of the same brand may not contain the same levels of active ingredient. In addition, the content of EPA and DHA may not match the amount indicated on the label.

Potential Side Effects

One of the main concerns associated with eating large quantities of fish is that some species contain high levels of pollutants like methylmercury and polychlorinated biphenyls (PCBs). One recent study suggested that methylmercury exposure may increase the risk of CHD, while another found that methylmercury did not increase the risk.

Given the current data, experts say that the potential benefits of eating fish outweigh the possible risks for middle-aged and older men and for postmenopausal women. If you are concerned about exposure to pollutants, try to eat a variety of fish and avoid species like shark, swordfish, king mackerel, and tilefish. Fresh tuna is also high in methylmercury, and large amounts should probably be avoided; but canned tuna appears to be relatively safe.

Fish oil supplements are mercury free, but they can cause side effects such as nausea, gastrointestinal upset, and a fishy aftertaste. High intakes of omega-3 fatty acids from fish oil supplements (more than 3,000 mg per day of EPA plus DHA) are associated with an increased risk of bleeding and should be taken only under a doctor’s supervision.


Good Sources of Omega-3 Fatty Acids
(EPA and DHA)


Type of fish
EPA + DHA
per 3 oz. serving
   

Herring

Sardines

Salmon

Mackerel

Trout

Halibut

Oysters

Tuna

Flounder

King crab

Shrimp

Clams

Lobster

Haddock

Cod

Catfish

Scallops

 

1,760 mg

1,340 mg

1,105 mg

955 mg

910 mg

700 mg

670 mg

585 mg

420 mg

350 mg

270 mg

240 mg

240 mg

200 mg

185 mg

180 mg

170 mg

Source: United States Department of Agriculture.

 

 


 


 

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The Heart Bulletin is a quarterly publication that presents the latest information available to help you make informed decisions about your cardiac care.
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2005
WHITE PAPERS
Coronary Heart Disease

The Coronary Heart Disease White Paper from The Johns Hopkins White Papers series is an annual, in-depth report written by Johns Hopkins physicians.

 

 

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